Generally, polymer materials and materials formed from polymers are sometimes classified in one of two groups, i.e., hydrophilic or hydrophobic, based upon the polymer surface affinity for water. If the polymer is water wettable or the polymer absorbs water or in someway unites with or takes up water, then the polymer is considered "hydrophilic". If the polymer is not water wettable or repels water or in someway does not unite with or absorb water, then the polymer is considered "hydrophobic".
While the water affinity property of a polymer is an important factor when determining the usefulness or applicability of a particular polymer in the formation of a product from such polymer, other factors, such as, but not limited to, costs, availability, polymer synthesis, environmental concerns, ease of handling, and current product composition are also weighed. In some instances, it may be more feasible to employ a water repellant or hydrophobic polymer in a product designed to absorb water than to use a water absorbent or hydrophilic polymer. In other instances it may be more feasible to employ a water absorbent or hydrophilic polymer in a product designed to repel water than to use a water repellant or hydrophobic polymer. In these instances, the selected polymer or polymer surface must be modified to conform to the intended use of the polymer in the ultimate product.
Historically, the surfaces of polymer compositions have been modified by non-permanent and permanent means. In the case of polymer compositions having hydrophobic surfaces, such non-permanent means include treating the polymer composition with surface active agents or surfactants. The surfactant, in combination with a hydrophilic composition, transforms the hydrophobic surfaces of the polymer composition to hydrophilic surfaces. However, in some instances, these modified hydrophilic surfaces generally became altered or lose their hydrophilic properties upon the first water wetting.
Permanent means of modifying polymer compositions include a number of wet chemical techniques and radiation techniques which initiate a chemical reaction between the polymer and a water affinity altering material. The process of chemical bonding by radiation is sometimes referred to as "grafting".
To permanently modify the polymer a chemical bond is formed between the polymer and the water affinity modifying material. Once chemically bonded to the polymer, the modifying material generally survives a first wetting so that the presence of the modifying material in the polymer alters the water affinity properties of the polymer for an extended period of time.
Wet chemical techniques include, but are not limited to, oxidation, acid or alkali treatments, halogenation and silicon derivative treatments. Radiation techniques which produce free radicals in the polymer include, but are not limited to, plasma or glow discharge, ultraviolet radiation, electron beam, beta particles, gamma rays, x-rays, neutrons and heavy charged particles.
There are generally three main techniques of forming chemical bonds between a polymer and a modifying material by radiation. These are: mutual; pre-radiation; and peroxide formation.
The mutual technique involves radiation of the polymer in the presence of a modifying material, wherein the modifying material is in either a liquid or vapor phase. The pre-radiation method involves radiating the polymer alone and then bringing the radiated polymer in contact with a modifying material, wherein the modifying material is in either a liquid or vapor phase. The peroxide formation method involves radiating the polymer in air and later decomposing the resulting polymeric peroxides in the presence of a modifying material, wherein the modifying material is in either a liquid or vapor phase. Both the mutual and the peroxide methods lead to homopolymerization, whereas the pre-radiation method leads mainly to copolymerization, i.e., chemical bonding occurring mainly between the polymer and the modifying material.
Generally, the mechanism by which a modifying material, and particularly a modifying material having a vinyl moiety, chemically bonds to the polymer upon radiation is via a free radical reaction between the polymer and the modifying material. Radiation produces free radicals in the polymer. The chemical bonding occurs in the polymer at the site of the formed free radical(s) and between the free radical(s) and the modifying material, and particularly between the free radical(s) and the modifying material's vinyl moiety.
Many of these radiation techniques have reaction times in minutes to hours to days. For example, ultraviolet radiation reactions using photoactivators have reaction times in the seconds to minutes range. Gamma radiation results in reaction times in the hours to days range. Additionally, many of these radiation techniques and wet chemical techniques may be relatively expensive and/or present environmental concerns.
In the case of on-line processing with web through-puts of between 600 to 1500 ft/min, rapid reaction rates are desirable. Therefore, there is a need to form chemical bonds with the polymer, such as on the surface of the polymer, in relatively short periods of time, generally less than 1/100 of a second, wherein the formation of such chemical bonds is economical, environmentally friendly and wherein the resulting bonds are sufficiently strong to survive multiple wettings.
To form chemical bonds under these requirements a sufficient dose of energy must be delivered to the polymer in less than 1/100 of a second. Of the above mentioned techniques, electron beam radiation affords the best opportunity to meet the rapid reaction times required by such on-line processing.
However, traditional on-line polymer processing via electron beam radiation at conventional energy levels of between 150 KV to 300 KV, producing electron energies greater than 145 KeV at the surface of the target polymer, has shown loss of polymer strength. Additionally, conventional electron beam radiation techniques, whether on-line or static, do not provide an ability to control the rate and location of the chemical bonding on the polymer. Furthermore, such conventional electron beam techniques deposit a large portion of the electron energy produced at these levels in the equipment shielding and not in the target polymer. Thus there exists a need to form chemical bonds by radiation, and particularly electron beam radiation, which avoids the above disadvantages.